IMHO the best option would be to export a list of forwarders, then use a script to read them sequentially from a file and push requests to the directadmin API.
The curl command you need is the following (replace the uppercase bits - save for CMD_API_EMAIL_FORWARDERS - with your own values):
curl -s --max-time 5 -u DAUSERNAME:DAPASSWORD/DALOGINKEY https://SERVER.mxrouting.net:2222/CMD_API_EMAIL_FORWARDERS -d 'json=yes&action=create&domain=MYDOMAIN.TLD&user=MYFORWARDER&email=RECIPIENT@MYDOMAIN.TLD'
A for-loop should do the trick.
Another option would be to ask Jarland (nicely) if he could import a .csv file.
As per the transition, IMHO:
- Set all the forwarders
- Move all the emails over via imapsync, imapcopy or similar toosl. This will allow you to see only new messages which could be delivered on the old host during the transition.
- Set the DNS properly
- After 12/24 hours move any message that could have been delivered to the old mailboxes
I hope this helps.